In their earliest stages most cancers are clinically silent. Patient diagnosis typically involves invasive procedures that frequently lack sensitivity and accuracy. Highly reliable, non-invasive screening methods would permit easier patient screening, diagnosis and prognostic evaluation.
Tumour derived markers are biological substances that are usually produced by malignant tumours. Ideally a tumour derived marker should be tumour-specific, provide an indication of tumour burden and should be produced in sufficient amounts to allow the detection of minimal disease. Most tumour derived markers used in clinical practice are tumour antigens, enzymes, hormones, receptors and growth factors that are detected by biochemical assays. The detection of DNA alterations such as mutations, deletions and epigenetic modifications (Baylin et al., 2000) provide another means for identifying cancers.
An epigenetic modification can be described as a stable alteration in gene expression potential that takes place during development and cell proliferation, mediated by mechanisms other than alterations in the primary nucleotide sequence of a gene. It is now general knowledge that both genetic and epigenetic alterations can lead to gene silencing and cellular dysfunction. Synergy between these two processes drives tumor progression and malignancy. Three related mechanisms that cause alteration in gene expression are recognised: DNA methylation, histone code changes and RNA interference.
DNA hypermethylation is an epigenetic modification whereby the gene activity is controlled by adding methyl groups (CH3) to specific cytosines of the DNA. In particular, methylation occurs in the cytosine of the CpG dinucleotides (CpG islands) which are concentrated in the promoter regions and introns in human genes (P. A. Jones et al., 2002; P. W. Laird et al., 2003). Methylation is associated with gene silencing. DNA hypermethylation is found to be involved in a variety of cancers including lung, breast, ovarian, kidney, cervical, prostate and also colorectal cancer. Methylation patterns of DNA from cancer cells are significantly different from those of normal cells. Therefore, detection of methylation patterns in appropriately selected genes of cancer cells can lead to discrimination of cancer cells from normal cells, thereby providing an approach to early detection of cancer.
DNA tumour markers, in particular DNA methylation markers, offer certain advantages when compared to other biochemical markers. An important advantage is that DNA alterations often precede apparent malignant changes and thus may be of use in early diagnosis of cancer. Since DNA is much more stable and, unlike protein, can be amplified by powerful amplification-based techniques for increased sensitivity, it offers applicability for situations where sensitive detection is necessary, such as when tumour DNA is scarce or diluted by an excess of normal DNA (Sidransky et al., 1997). Bodily fluids provide a cost-effective and early non-invasive procedure for cancer detection. In this context, faecal-based cancer testing has been one area of investigation.
Human colorectal cancer has provided a good model for investigating whether DNA cancer markers can be adopted as an optimal faecal-based diagnostic screening test. Central to faecal-based colorectal cancer testing has been the identification of specific and sensitive cancer derived markers.
The N-Myc downstream-regulated gene (NDRG) family comprises four family members: NDRG1 (NDRG-family member 1), NDRG2 (NDRG-family member 2), NDRG3 (NDRG-family member 3) and NDRG4 (NDRG-family member 4). The human NDRG1 and NDRG3 belong to one subfamily, and NDRG2 and NDRG4 to another. At amino acid (aa) level, the four members share 53-65% identity. The four proteins contain an alpha/beta hydrolase fold as in human lysosomal acid lipase but are suggested to display different specific functions in distinct tissues.
NDRG1 codes for a cytoplasmic protein believed to be involved in stress responses, hormone responses, cell growth, and cell differentiation. NDRG1 has been demonstrated to be upregulated during cell differentiation, repressed by N-myc and c-myc in embryonic cells, and suppressed in several tumor cells (Qu X et al., 2002; Guan et al., 2000).
NDRG3 is believed to play a role in spermatogenesis since it is highly expressed in testis, prostate and ovary (Zhao W et al., 2001). Its involvement in brain cancer development has also been suggested (Qu X et al. 2002).
NDRG2 codes for a cytoplasmic protein that seems to be involved in neurite outgrowth and in glioblastoma carcinogenesis (Deng Y et al., 2003). It is upregulated at both the RNA and protein levels in Alzheimer's disease brains (Mitchelmore C et al., 2004), and has also been suggested to play an important role in the development of brain cancer (Qu X et al. 2002), pancreatic cancer and liver cancer (Hu X L et al., 2004).
The NDRG4 cytoplasmic protein is involved in the regulation of mitogenic signalling in vascular smooth muscles cells (Nishimoto S et al.). The NDRG4 gene contains 17 exons, and several alternatively spliced transcript variants of this gene have been described. NDRG4 may also be involved in brain cancer development (Qu X et al. 2002).
Suppressed expression of NDRG-family genes has been demonstrated in a number of tumours (Qu X et al. 2002) and the involvement of DNA promoter hypermethylation is limited to the reporting of NDRG2 methylation in brain tumors (Lusis et al., 2005).
Initially, faecal-based DNA assays investigated the usefulness of specific point mutations markers for detecting colorectal cancer. Later, the DNA integrity in faecal samples proved to be a useful marker (Boynton et al., 2003). Finally, faecal testing based on DNA alterations gradually evolved into the development of a multi-target DNA assay using specific point mutation markers, a microsatellite instability marker and a marker for DNA integrity. Recently, the potential of faecal DNA testing targeting epigenetic alterations has been investigated (Müller et al., 2004, Chen et al., 2005) and has been added to the multi-target DNA assay. Genes having an altered methylation status traceable in faecal DNA from colon cancer patients versus control samples from healthy subjects have been discovered (Belshaw et al., 2004; Petko et al., 2005; Lenhard et al., 2005; Müller et al., 2004; Chen et al., 2005 and Lueng et al., 2004).
Factors that may influence the sensitivity of the selected markers are sampling processing procedures and DNA isolation and extraction protocols. One challenge faced by researchers investigating colorectal cancer is the diversity of DNA present in stool samples. Most of the DNA recovered from faecal samples is bacterial in origin, with the human DNA component representing only a very small minority. Human DNA from cells sloughed from the colonic mucosa represents as little as 0.1 to 0.01% of the total DNA recoverable from stool. Additionally, the human DNA recovered is highly heterogeneous. Normal cells are sloughed into the colonic lumen along with only a small amount of tumour cells (approximately 1% of the cells sloughed). Thus, the DNA of interest represents only a very small percentage of the total DNA isolated from stool. Therefore, along with the exploration of suitable DNA markers, techniques for improved DNA isolation and enrichment of the human DNA component from faecal samples have been developed for more sensitive cancer detection.
The initial DNA isolation techniques typically recovered DNA from 10 g to 4 g stool and more conveniently purified the human DNA component using streptavidin-bound magnetic beads (Dong et al., 2001; Ahlquist et al., 2000). Further improvements in recovery of target human DNA from stool comprised an electrophoresis-driven separation of target DNA sequences, using oligonucleotide capture probes immobilized in an acrylamide gel (Whitney et al., 2004). Later, when DNA integrity proved to be a suitable marker it was also important to prevent degradation during sample handling. Improved results were obtained with stool samples frozen as quickly as possible after collection. Alternatively, stabilization buffer was added to the stool samples before further transport (Olson et al., 2005). A recent improvement involves the use of an MBD column to extract methylated human DNA in a high background of fecal bacterial DNA (Zou et al., 2007). However, despite these advances, current tools for cancer detection in faecal samples are still unsatisfactory.
Cancer at its early stage may release its cells or free DNA into blood through apoptosis, necrosis or local angiogenesis, which establishes a basis for blood-based cancer testing. The usefulness of DNA methylation markers for detecting colorectal cancers in serum and plasma has been demonstrated (Grady et al., 2001, Leung et al., 2005; Nakayama et al., 2007). However, the potential use of serum and plasma for cancer detection is hampered by the limited level of methylated DNA present in the total DNA collected from plasma and serum samples (Zou et al. (2002) Clin Cancer Res 188-91). A further drawback is the partial degradation of the methylated DNA due to bisulfite treatment, a treatment step required by many techniques that monitor DNA methylation.
Methods and compositions for detection of early colorectal cancer or pre-cancer using blood and body fluids have been described.
WO 2006/113770 describes methods in which samples are pooled and concentrated in an attempt to maximize DNA input per reaction. The initial processing of 45 ml of blood allowed a median DNA recovery of 3.86 ng/ml plasma. This was shown to result in a sensitivity of 57% and specificity of 96% for detection of colorectal cancer using a specific real-time assay for detecting whether The Septin 9 gene was methylated. Bisulphite treatment was focused on large volume treatment and achieving maximal conversion.
Lofton-Day et al. (AACR general meeting April 2007, Los Angeles, USA) mention improved detection of colorectal cancer, and obtained a 70% sensitivity and 90% specificity, with the same marker (Septin 9). The proposed method utilised four blood draws (40 ml blood), double centrifugation for plasma recovery and required four PCR reactions to be carried out for each sample tested. Three out of the four reactions used input DNA equivalent to 2 ml of plasma per PCR reaction. The fourth reaction used a 1/10 dilution of this input DNA. Thus, repeated assays were required (at least 4) and an algorithm utilised to determine the final result. A sample was deemed positive if either two out of the three reactions with input DNA equivalent to 2 ml of plasma, or the diluted measurement, were positive for the Septin 9 assay. The improved sensitivity by using the diluted samples indicates the presence of inhibitors in the methods, a phenomena also described by Nakayama et al. (2007, Anticancer Res. 27(3B):1459-63).
The processing of smaller amounts of blood have been described as well (US 20070141582, Hong-Zhi Zou et al., and Satoru Yamaguchi et al.) but all result in low level of methylated modified DNA detection.
Thus, current blood-based screening methods lack sensitivity.